This invention relates to pulsed energy detectors useful in the spectral range from ultraviolet to far infrared. More particularly, this invention relates to pyroelectric joulemeters used to measure and calibrate the output of pulsed energy devices.
Pyroelectric joulemeters are known which are used to measure the energy output of pulsed lasers. A typical pyroelectric joulemeter includes a thin rectangular or circular pyroelectric detector wafer provided with front and rear surface electrodes for manifesting electrical signals generated by the pyroelectric effect when the detector wafer is subjected to pulsed incident radiation. Typical pyroelectric materials known in the art are lithium niobate, barium strontium niobate, lithium tantalate, lead zirconate titanate (PZT). lantham-doped lead zirconate titanate, thalium arsenic selenide, and polyvinylidene fluoride (PVDF) The detector element is normally mounted within a protective housing, with one face of the detector exposed to ambient, either directly or through a protective window plate. An example of a known prior art pyroelectro detector is shown in U.S. Pat. No. 3,571,592,
Pyroelectric joulemeters may be generally classified as small area detectors and large area detectors. The small area detectors, which typically employ a pyroelectric detector wafer having a diameter in the range from about 2 to 10 mm., are especially suited for use in measuring and calibrating the pulse energy output of lasers having energies in the microjoule to millijoule range. Such small area devices typically require an electronic amplifier incorporated into the detector device due to the small detector internal capacitance, which is typically less than the capacitance of the cable used to connect the detector to a measuring instrument, typically an oscilloscope. The requirement for an amplifier adds cost to the device. manufacturing complexity, and introduces electronic noise into the pyroelectric signal generated by the detector, all of which are disadvantageous. In addition, small area detectors having an internal amplifier typically require a special termination element in order to match the input impedance of the measuring instrument.
Large area detectors are designed for use with pulsed lasers having energies in the millijoules to joules range and, as suggested by the name, use a pyroelectric detector wafer which is much larger in area than the small area detectors and typically has a diameter in the range from about 25 to 50 mm. Some commercially available large area detectors use a built in preamplifier, with the same attendant disadvantages as those noted above for the small area detectors. Other commercially available large area detectors do not employ an internal amplifier, but suffer from several disadvantages, such as poor sensitivity. microphonic noise generation (due to the piezoelectric effect inherent in pyroelectric materials) and relatively poor heat dissipation, which can lead to premature failure of the detector element. In addition, known large area detectors suffer from a relatively low maximum repetition rate, typically about 2 pulses per second for a detector with a 50 mm pyroelectric detector element. This relatively low maximum repetition rate limits the usefulness of such devices in measuring and calibrating pulsed laser outputs and is due primarily to the relatively high dielectric coefficient of the pyroelectric material used in some commercially available detectors and the thermal characteristics of the detector element and any coatings applied thereto. Another disadvantage with known large area detectors is the relatively high sensitivity of the pyroelectric coefficient to temperature changes. For PZT this sensitivity factor is 1.2% per degree Kelvin, while for PVDF this factor is 0.5% per degree Kelvin. Since the detector element can be subjected to wide temperature swings at a relatively fast rate, the pyroelectric coefficient can also vary widely with temperature changes, which adversely affects the output signal from the device.
In addition, most large area pyroelectric joulemeters employ a sealed unit construction, which requires transport of the entire joulemeter to the manufacturer for repair. As a consequence, the user is virtually forced to purchase two complete units--a main unit and a backup unit--to maintain constant joulemeter measurement and calibration capability. Still further, the voltage responsivity, measured in volts per joule,of known large area detectors can vary from unit to unit, which is undesirable.
Efforts to date to design a large area pyroelectric joulemeter devoid of the above-noted disadvantages have not met with success to date.